Energy & Fuels 2003, 17, 1175-1179
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Comparison of Improved Combustion/Trap Technique to Wet Extraction Methods for Determination of Mercury in Crude Oil and Related Products by Atomic Fluorescence L. Liang,*,† M. Horvat,‡ V. Fajon,‡ N. Prosenc,‡ H. Li,† and P. Pang† Cebam Analytical, Inc., 3927 Aurora Avenue N., Seattle, Washington 98103, and Department of Environmental Sciences, J. Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia Received February 25, 2003. Revised Manuscript Received June 4, 2003
Because trace amounts of mercury in crude oil can produce negative impacts on refinery operations and profitability, and thus affect oil’s quality and price, accurate determination of mercury using a reliable technique is very important. Since its development in 1995, the combustion/trap technique has been continuously improved and employed to generate accurate results for the refinery industry, and showed significant advantages over wet extraction methods. The analytical performance of the combustion/trap technique in different types of samples at different mercury concentration levels was statistically compared with wet extraction methods. The statistical comparison would help refiners and analytical chemists in understanding and choosing an adequate technique for analysis of crude oil and various refinery products, and help in assessing existing analytical results obtained using different techniques. Comparison of results indicated that wet extraction methods could generate negative bias of up to 50%, depending on different types and concentration levels of samples. Negative bias found for oil samples can hide potential risks caused by mercury poisoning to some critical refining operations which require processing of oils free of mercury. The combustion/trap technique can be used for analysis of all crude oils and various refinery products except sulfur, while wet extraction methods cannot be used for analysis of most refinery products.
Because mercury can deposit in equipment, resulting in important health and safety issue during inspection and maintenance operations, to predict the impact of particular oils on refining, and to determine whether atmospheric emissions of mercury from petroleum-based fuels are significant or not for compliance of environmental regulatory limits, refiners need to know exactly how much Hg is in the refinery crude diet.1,2 However, crude oil and refinery byproducts are difficult to analyze for mercury concentrations using previously existing techniques.3 Thus, development and improvement of reliable methods for analysis of mercury in these refractory samples is important and necessary. There were various existing wet extraction methods combining different detection techniques such as atomic absorption (AA), induced coupled plasma (ICP), and ICP-mass spectrometry (ICP-MS). Due to some drawbacks and problems,3 these previously existing methods could not accurately detect low-level Hg concentrations in refinery samples. Some studies have been conducted
on developing and applying analytical methods based on sensitive atomic fluorescence detection technique since 1993, and one of the methods was published for the first time in 1996.3 Since then, several analytical methods based on the same fluorescence detection technique for determination of mercury in crude oil and related products and refining process byproducts have been published.4-6 Among these currently developed techniques, the combustion/trap technique showed significant advantages. The combustion/trap technique does not require sample preparation, therefore positive or/and negative bias generated during sample preparation can be eliminated, resulting in more precise and accurate results. Moreover, the technique makes it possible to accurately determine mercury in solid-phase refinery products such as coke, resin, asphalt, acid sludge, sulfur, and wax. Because these samples are acid-resistant products, and some of them contain Hg at levels near or lower than method detection limits of wet acid extraction methods, mercury in these samples could not be extracted quan-
* Corresponding author. Tel.: (206) 632 9097. Fax: (206) 632 1947. E-mail:
[email protected]. † Cebam Analytical, Inc. ‡ J. Stefan Institute. (1) http://www.hgtech.com/basic/EPA%20API%20Project.htm. (2) Wilhelm, S. M. Environ. Sci. Technol. 2001, 35 (24), 4704. (3) Liang, L.; Horvat, M.; Danilchik, P. Sci. Total Environ. 1996, 187, 57-64.
(4) Liang, L.; Horvat, M.; Danilchik, P. Novel analytical methods for determination of low level of total mercury in petroleum and its products by gold amalgamation CVAFS, the 4th International Conference on Mercury as a Global Pollutant, August 4-8, 1996, Hamburg, Germany. (5) Bloom, N. Fresenius J. Anal. Chem. 2000, 367, 8-11. (6) Liang, L.; Lazoff, S.; Horvat, M.; Swain, E.; Gilkeson, J. Fresenius J. Anal. Chem. 2000, 367, 8-11.
1. Introduction
10.1021/ef030042j CCC: $25.00 © 2003 American Chemical Society Published on Web 07/23/2003
1176 Energy & Fuels, Vol. 17, No. 5, 2003
titatively into aqueous solutions with BrCl/HCl3 or using traditional wet digestion methods. The combustion/trap technique has been used for the analysis of refinery industry samples for several years. The focus of this paper is to report the improvements made on the technique over the past several years, and to compare its performance statistically in different types of samples, at different mercury concentration levels with wet extraction methods. The information provided in this paper may greatly help refiners, analytical chemists, and laboratories in choosing an adequate analytical method for determination of lowlevel mercury in refinery samples, and in assessing existing results obtained using various analytical methods. 2. Experimental Section 2.1. Instrument and Materials. A fluorescence spectrophotometer (Brooks Rand BRIII) was used for the detection of elemental Hg. The combustion/trap system and other materials used were the same as those described previously6 with the following improvements. A combustion column of 5.79 mm i.d. and 7.98 mm o.d. was used, the carrier air flow rate was 320 mL/min measured at the end of a gold sand trap, and both segments of the combustion column were packed with quartz wool. A flow meter (Humonics Digital Flow Meter 520) was used for measuring air flow rates of the system. Methyl mercury stock solution was prepared in 2-propanol as described in our previous paper. In this work, working solutions were prepared by serial dilution of the stock solution with CH2Cl2 which was also used for dilution of oil samples. 2.2. BrCl/HCl Extraction Procedure for Analysis of Crude Oil and Other Liquid Oil Samples. Details of this procedure were provided previously.3 Some modifications to the procedure were made. Briefly, 4.0 mL of oil or diluted sample were pipetted into a 40-mL glass vial with a Teflonlined cap, and 6.0 mL of BrCl and 6.0 mL of concentrated HCl were added. The BrCl solution was prepared according to EPA method 1631.7 The sample was shaken vigorously for 30 min for extraction. After extraction, an aliquot (0.1 to 3.0 mL) of the aqueous phase was pipetted into a bubbler for analysis using modified EPA 1631, SnCl2 reduction, purge and trap, and CVAFS detection.7 Results were converted from ng/mL into ng/g for reporting using oil densities. For viscous oil samples, sealed sample bottles were heated to around 60 °C, then shaken vigorously for 2 min. Between 1 and 2 g (to the nearest mg) of a sample was weighed into a glass vial, and toluene (or other light solvents with low Hg blanks) was added to bring the volume to 4 mL; the sample was then treated as described above for extraction and analysis. 2.3. Acid Digestion Procedure for Analysis of Solid Refining Products. Approximately 1 g of sample was digested with 10 mL of HNO3/H2SO4 (7/3) in a 60-mL Teflon vial for 3 h at 95 °C. The digestate was further oxidized with 0.5 mL BrCl. The volume of digestate was brought to a known volume with double-deionized water (DDW), then analyzed using a modified EPA 1631 method.7 2.4. Na2S Dissolution-Acid Digestion Procedure for Analysis of Sulfur Samples. Approximately 1 g of a sample was first dissolved in 4 mL of saturated Na2S in a 60-mL Teflon vial, then 10 mL of concentrated HNO3 was added slowly, and the vial was heated at 95 °C for 3 h. The digestate was further oxidized with 0.5 mL of BrCl. The volume of digestate was brought to a known value with DDW, then analyzed using a modified EPA 1631 method.7 (7) EPA Method 1631.
Liang et al.
Figure 1. Effect of air flow rate on atomization rate of combustion. 2.5. Combustion/Trap Procedure. Details of the analysis of samples were provided previously.6
3. Results and Discussion 3.1. Major Improvement of the Technique. Many factors could affect the atomization rate,6 our studies indicated that the most important one was the air flow rate. In this work, air flow rates were measured by connecting a flow meter (Humonics Digital Flow Meter 520) to the end of the gold sand trap which was connected in-line to the combustion system for collection of generated Hg(0). A sample of 1 ng of methyl mercury standard was injected to the combustion system for analysis at different flow rates, while 1 ng of Hg2+ standard was analyzed by SnCl2 reduction in 6 replicates. The mean response of the analyzer for 6 replicate analyses by SnCl2 reduction was assumed to be full atomization (100%). Our results indicate that the atomization rate represented as percentage increases with increasing air flow rate (Figure 1), and an atomization rate of 100% was achieved at air flow rates above 300 mL/min. Analytical precision and accuracy were also improved at higher flow rates. Two factors, the column cross-sectional area and packing density, may affect the air flow rate. We cannot measure or control the packing density. However, as described above, the air flow rate can be precisely measured by connecting a flow meter (Humonics Digital Flow Meter 520) to the end of the gold sand trap of the system, therefore allowing the flow rate to be quantitatively controlled. This was a major improvement of the technique. In our previous work,6 the flow rates were incorrectly measured and actually lower rates were used resulting in lower atomization rates. 3.2. Comparison of Method Detection Limits (MDL) of the Combustion/Trap Method to Wet Chemistry Methods for Analyses of Liquid Oil and Solid Samples. Determinations of the MDLs of the technique for oil samples were conducted by analyzing 9 replicates of a low-concentration oil sample. Various low-level samples including gasoline, diesel, kerosene, and crude oil were analyzed. The MDLs ranged from 0.05 to 0.12 ng/g. The MDLs for solid samples were determined by analyzing 8 replicates of a coke sample with Hg concentration around 0.5 ng/g. The MDL was found to be around 0.15 ng/g. The Coke sample was chosen for determinations of MDL for solid samples
Determination of Hg in Crude Oil and Related Products
Energy & Fuels, Vol. 17, No. 5, 2003 1177 Table 1. Summary of Results for Replicate Analyses of Hg in a Crude Oil Sample Using the Combustion/Trap and BrCl/HCl Extraction Methods number of replicate analysis, n
mean, ng/g
SD, ng/g
RSD, %
11 11
86.3 81.7
2.5 3.0
2.9 3.6
combustion/trap BrCl/HCl extraction
Table 2. Comparison of Recoveries Generated by the Combustion/Trap and BrCl/HCl Extraction Methods for Analysis of Oil Samples with Various Hg Concentration Ranges
Figure 2. Comparison of results of analysis of Hg in oil samples by combustion/trap and BrCl/HCl extraction, Hg concentration range: 0.1-2.5 ng/g.
Figure 3. Comparison of results for analysis of Hg in oil samples by combustion/trap and BrCl/HCl extraction, Hg concentration range: 3-20 ng/g.
because they were known to be homogeneous and low in Hg. Apparently, no traditional wet extraction methods could generate MDLs as low as those by combustion/ trap technique. 3.3. Comparison of the Combustion/Trap Technique and BrCl/HCl Extraction for the Analysis of Hg in Oil Samples. Statistically significant differences were found in comparing the results obtained from the combustion/trap technique and BrCl/HCl extraction methods. Since the magnitude of these differences appeared to correlate with the Hg concentrations of oil samples, the results were grouped and compared in several concentration ranges. Figure 2 shows the results for 22 oil samples (including gasoline, kerosene, diesel, and crude oils) with Hg concentrations ranging from 0.1 to 2.5 ng/g. Figure 3 shows the results for 21 crude oil samples with Hg concentrations ranging from 3 to 20 ng/g. All samples were analyzed in duplicate using each method, and the mean results of duplicate analyses are shown. The RSDs of duplicate analyses were below 7%. We had too few samples with higher Hg concentration to conduct the same analyses shown in Figure 2 and Figure 3. Therefore, a crude oil sample with a Hg concentration around 86 ng/g was analyzed in 11 replicates using the two methods; the results are summarized in Table 1. To compare the results directly, statistical recoveries for each method were calculated using the slopes of regression equations in Figure 2 and Figure 3, and mean values in Table 1; these are listed in Table 2. Recoveries of the BrCl/HCl extraction method in Table 2 were calculated assuming that recoveries of the combustion/ trap method were 100%. The data in Table 2 show that recoveries obtained using the two methods were within the general ac-
Hg concentration range, ng/g
number of samples analyzed, n
0.1-2.5 3-20 86
22 21 11
recovery, % combustion/ BrCl/ trap HCl extraction 100 100 100
85.0 91.7 94.7
ceptance criteria of 75-125%. There were, however, statistically significant differences in the results from the two methods, with Hg concentrations determined by the combustion/trap method higher than those determined by BrCl/HCl extraction for all concentration ranges investigated. The differences may represent a systematic bias between the two methods. The working standards for both methods were made from the same standard solution of 1.00 µg/mL methyl mercury as Hg in 2-propanol, traceable to NIST standards. Therefore, a difference of standards used by the two methods could not have caused the observed discrepancy in results. Since the combustion/trap technique omitted sample preparation steps, and no significant contamination was made during analyses, we attributed the deviations between the two methods to incomplete extraction of Hg from oil samples by BrCl/HCl extraction, or over-correction for blanks mostly from BrCl reagent, or both. Additionally, when the BrCl/HCl extracts are analyzed by SnCl2 reduction, purging, and gold sand trap collection of elemental Hg, free halogens generated from the reaction between SnCl2 and BrCl may damage gold traps, subsequently decreasing the trapping efficiency and lowering the results. Recoveries lower than 50% could be encountered when old traps were used. It is worth noting that in Table 2, all results by BrCl/HCl extraction were obtained using new gold traps, i.e., no traps used more than 6 times were employed for this comparison work. However, it would not be practical to request a laboratory to keep using new traps for routine analyses because gold sand traps are expensive. These sources of error would affect the results of low-concentration samples more significantly compared to higher-concentration samples. This would explain the data in Table 2 where recoveries by BrCl/ HCl extraction decrease with decreasing Hg concentration. 3.4. Comparison of the Combustion/Trap Technique and Acid Digestion for the Analysis of Hg in Solid Refining Products. Mercury is a contaminant in a number of refining products such as coke, resin, asphalt, acid sludge, sulfur, and waxes, and many chemicals produced from these products. Moreover, there are many materials and chemicals used for thermal refining, catalytic refining, isomerization, alky-
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Figure 4. Comparison of results between the combustion/trap technique and the acid digestion method for determination of Hg in coke samples.
Figure 5. Comparison of results between the combustion/trap technique and the acid digestion method for determination of Hg in sulfur samples.
lation, and polymerization in the petroleum industry8 that need to be analyzed for Hg. Most of these solid materials are acid resistant, i.e., it would be difficult to put these samples in solution either by traditional acid digestion or by BrCl/HCl extraction. As a result, few of these materials would be suitable for a comparison test between acid digestion and the combustion/trap technique. Coke and sulfur were ultimately chosen as suitable test materials. In the determination of Hg in coke samples, results using the combustion/trap technique were almost double those obtained by the acid digestion method (Figure 4). Concentrations of Hg in the coke samples ranged from 0.1 to 0.6 ng/g; thus, the acid digestion method, with an MDL around 0.5 ng/g, was not appropriate for this analysis. The combustion/trap technique thus showed significant advantages for low-concentration samples. The data in Figure 5 show comparison results between the two methods for analysis of Hg in sulfur samples. For sulfur analysis, in addition to acid digestion, samples were also digested using a special procedure. Sulfur samples were first dissolved in a saturated Na2S solution, then digested with HNO3, and further oxidized with BrCl. When HNO3 was added, a strong reaction occurred while Hg was extracted into acid solution. Results generated using this procedure were compared with those generated by the combustion/trap technique (Figure 6). The data show that results from the Na2S dissolution-acid digestion are almost three times those obtained using the combustion/trap technique. What accounts for this discrepancy? All spike recoveries associated with the analyses were acceptable. However, acceptable spike recoveries do not necessarily mean the results are correct in this case. The Hg in the standard spiked into the sulfur sample was different in nature from that occurring naturally in the sulfur. Sulfur is a complex mixture of S2, S4, S6, and S8. The (8) Speight, J. G. The Chemistry and Technology of Petroleum, 3rd ed.; Marcel Dekker: New York.
Liang et al.
Figure 6. Comparison of results between the combustion/trap technique and the Na2S dissolution-acid digestion for determination of Hg in sulfur samples.
exact nature of the Hg species formed in association with this sulfur complex is unknown.Whereas the Hg spike was easily extracted by the acid, the Hg occurring naturally in the sulfur may not have been. Although an agreement of results between the combustion/trap technique and acid digestion was achieved, results obtained using these two methods were not necessarily accurate because of the possibly incomplete extraction of naturally occurring Hg in sulfur. Unlike other samples, the analysis of Hg in sulfur using the combustion/trap technique was complex. In addition to combustion, another process, sublimation of sulfur, took place at the same time in the combustion column. It was also observed that sublimated sulfur condensed rapidly on the cold end of the combustion column. If the sample introduced into the column was small, and the temperature of the heating wire applied to the introduction segment of the column increased rapidly, the combustion process would dominate, resulting in higher results. Otherwise, sublimation would take place, resulting in lower results. Sublimated sulfur could carry bound Hg and condense on the cold end of the combustion column. Thus some Hg occurring in sulfur was carried through the column and not converted into elemental Hg. If the process could be controlled so that only combustion would take place, the Hg occurring in sulfur would be recovered completely. The combustion technique should not be used for sulfur samples until this issue is resolved. Some sulfur samples were also analyzed using the RNAA method,9 but the MDL for Hg in sulfur was found to be far above the concentration range of Hg. Considering all of these results, we would recommend the Na2S dissolution-acid digestion followed by modified EPA 1631 for analysis of Hg in sulfur samples until appropriate techniques are developed. Note that blanks of the Na2S reagent were found to be higher, so the results must be blank-corrected. 3.5. Standardization. Standard solutions of different Hg species (organo-Hg including methyl Hg and phenyl Hg; and inorganic Hg including HgCl2 and HgS) have been compared, and responses of the detector were found to be independent of Hg species examined. This indicates that atomization rate of different species were identical, thus, results generated using the technique are independent of Hg species. This ensured the reliability of the technique for analyzing oil samples from various sources because oil samples from various sources contain different Hg species. The HgS standard was prepared by dissolving HgS powder into saturated Na2S solution. (9) Byrne, A. R.; Kosta, L. Talanta 1974, 21, 1083.
Determination of Hg in Crude Oil and Related Products
Systematic positive deviations were observed for certified solid samples when analyzed using liquid standards for calibration. Although deviations were generally within (25%, it may suggest there are some differences in atomization rates. When certified solid materials were used for calibration, the positive deviations were eliminated, resulting in accurate results. All certified solid reference materials could be used as calibration standards. To minimize analytical deviations, standards that are chemically and physically similar to samples to be analyzed should be used. For instance, in this work, NIST2709 soil and NIST2704 sediment have been used as standards for the analysis of solid samples. The response of the fluorescence spectrophotometer to Hg standards loaded on gold traps but generated either by the combustion/trap system or the SnCl2 reduction bubbler7 was also compared. It was found that responses generated by the two methods were identical. Instrument responses of the combustion/trap technique do not need to be compared to those of SnCl2 reduction. However, since the SnCl2 reduction is a classic analytical method, using it as a reference method for evaluation of a new developed method would be meaningful.
Energy & Fuels, Vol. 17, No. 5, 2003 1179
4. Conclusions The combustion/trap technique is a sensitive and reliable method for the determination of Hg in various types of materials used or produced by the refinery industry. The method can be used for analysis of all hydrocarbon products. The method is particularly useful for samples with low Hg concentrations (
